This invention relates in general to a control system for a motor wherein a plurality of electromagnetic field windings are separately energized to control the rotational position and movement of a rotor, and/or apparatus driven by the rotor, and more particularly to a control system which detects back EMF induced as a result of rotor rotation, to develop a feedback signal useful for controlling operation of the motor or detecting the operating state of apparatus driven by the motor. The control system of the invention is particularly useful in controlling the operation of a print head positioning motor in a high speed teleprinter wherein it is necessary that the print head be positioned and controlled with a high degree of accuracy.
It has been common practice in the electric motor arts to use a tachometer or the like driven either directly or indirectly from the rotor shaft of a motor to determine the speed and other operational parameters of the motor. It has also been common practice to utilize the output signal from the tachometer in a feedback loop to control energization of the motor to achieve a desired standard of motor performance including the maintainance of uniform motor speeds with variations in load, and the optimizing of acceleration and deceleration characteristics of the motor.
More recently, other means of sensing motor speed and performance have come into use. For example, optical pickup systems, wherein rotation of an opaque disc with radially disposed transparent slots is sensed by means of a light source on one side of the disc and a photoelectric sensor on the opposite side of the disc, have been coupled to a motor rotor to detect motor rotation. Unfortunately, errors may be introduced by these optical pickups due to eccentricites in the disc or in the mounting of the disc to the rotor shaft. Even slight eccentricities of the slot in relation to the axis of the rotor shaft can cause the photoelectric sensor to detect false acceleration and deceleration for each rotation of the rotor shaft. Moreover, where such discs are used with stepping motors and the intertia of the load is small, the disc may undesirably add to the rotational inertia of the rotor and detract from the acceleration or deceleration performance of the motor. Furthermore, such optical detection systems undesirably add to the space required by the motor, and to the complexity and cost of the apparatus in which they are utilized.
Other techniques have previously been employed to attempt to recover and utilize the back EMF induced into a motor winding to detect and control performance of the motor. In one such attempt the back EMF was recovered by using an additional winding in the motor. However, since the additional sense winding had to be magnetically shielded from the field windings to avoid inducing drive signals into the sense windings, this approach undesireably added to the bulk, weight and cost of the motor.
It is therefore a primary object of the present invention to provide a new and improved motor control system wherein back EMF generated during operation of a motor is recovered and utilized to control the operation of the motor.
A more specific object of the present invention is to provide reactive coupling between inputs and outputs of a pair of operational amplifiers responsive to respective motor control signals to simulate the signal induced into an unenergized winding of the motor.
A further object of the present invention is to provide signal summing between the outputs of the operational amplifiers and a pair of inputs of a comparator amplifier such that the comparator compares a simulated signal from the electronic circuitry with the induced signals in each of a pair of motor field windings in generally 180.degree. phase relationship.
Yet another object of the present invention is to provide such circuitry for detecting the back EMF for each pair of electromagnetic field windings of the motor having mutual inductance therebetween.
A further object of the present invention is to provide a method of detecting the back EMF induced into an unenergized winding by rotational movement of the rotor, wherein the back EMF is superimposed upon another signal induced into the unenergized winding because of mutual inductance with an energized winding, including the steps of electronically simulating the signal induced into the unenergized winding because of mutual inductance with the energized winding, but with the simulated signal having no back EMF superimposed thereon, comparing the simulated signal to the signal induced into the unenergized winding with the back EMF superimposed thereon to detect the back EMF, and utilizing the detected back EMF to control subsequent energization of the motor field windings, to detect position of apparatus driven by the rotor, or to actuate such apparatus.